Author Topic: 30cc Flat Four (Puma)  (Read 9697 times)

Offline Jasonb

  • Full Member
  • *****
  • Posts: 6735
  • Surrey, UK
Re: 30cc Flat Four (Puma)
« Reply #195 on: September 29, 2019, 07:16:28 AM »
The other method is to put a full length prop on it and measure what revs it will do, there is then a formula that will give the HP.

Offline doubletop

  • Full Member
  • ****
  • Posts: 329
  • Wellington NZ
Re: 30cc Flat Four (Puma)
« Reply #196 on: September 29, 2019, 08:02:16 AM »

The other method is to put a full length prop on it and measure what revs it will do, there is then a formula that will give the HP.


https://www.rcgroups.com/forums/showthread.php?1077012-Measuring-motor-power-output

Apparently no prop is required, just a stick. Although I would be a bit wary of a 6mm x 26mm plywood 'stick' spinning at 8000 rpm, I'd have visions of it parting company with the engine and launching itself at the neighbours place. However, the material used is apparently irrelavant as mass has no part to play in the calculations.


Pete
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline tangler

  • Full Member
  • ****
  • Posts: 778
  • Christchurch, UK
Re: 30cc Flat Four (Puma)
« Reply #197 on: September 29, 2019, 11:48:14 AM »
There's a formula (of uncertain provenance) that gives Horse Power figures that are comparable to some engine tests that Peter Chinn used to publish.

BHP = 7.105 x 10-18 x D4 x P x R3

Where D is prop diameter, P is the Pitch (both in inches) and R is RPM

HTH,

Rod

Offline doubletop

  • Full Member
  • ****
  • Posts: 329
  • Wellington NZ
Re: 30cc Flat Four (Puma)
« Reply #198 on: September 30, 2019, 01:38:49 AM »
Rod

Thanks for the input, weíve started something hereÖ

I went looking for references to Peter Chinn to support the formula provided by your input. It turns out that Peter Chinn used his own set of calibrated props to get results. And if you read the articles in the links below all props are not equal. In Peter Chinnís time even the same prop from the same manufacturer wasnít equal

http://www.adriansmodelaeroengines.com/catalog/main.php?cat_id=57

http://www.modelenginenews.org/techniques/testing.html

Where does this take me? Back to letís get the engine running properly and maybe then Iíll try a Prony brake.

Pete

#Dynamometers


?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss

Offline doubletop

  • Full Member
  • ****
  • Posts: 329
  • Wellington NZ
Re: 30cc Flat Four (Puma)
« Reply #199 on: October 01, 2019, 06:32:09 AM »
I gave it another run and although there was an improvement over previous runs it was clear it still needed some work. One benefit of a multi cylinder engine is that if one cylinder isnít working as well as the others it is pretty obvious.

Iíd previously noted that one of the cams on one of camshafts were not correctly aligned, I was also aware that two of the cylinders had considerably more compression than the other two.

So time to strip it all down again and this time get the camshafts out. The jacking screws certainly helped to make the strip down a lot easier.



Youíll notice the cam shaft has been disassembled and the crank is missing. With it all apart this time the problem with one of the crank pins coming out was obvious, one of the joints on the centre web had failed. So that was the first thing to get fixed.

I checked the angular location of the cams and confirmed they were out. The right hand (2,4) camshaft the two exhaust cams should have been 90deg apart they werenít. Same for the inlet cams on the left hand side (1,3)

When Malcolm wrote up the Bobcat article, he proposed a simple jig to align the cams



Youíll notice that even in this photo it is possible to get the cam incorrectly located in the V of the jig. Any error on the camshaft alignment is doubled at the crankshaft.

I re-made my jig to better fit the shape of the cam to better ensure correct alignment.



Investigating the two cylinders with lower compression was an easy change of the O rings. I guess they wore more quickly doing the run in.

Loctite is now curing and Iím cleaing up the other parts. Then its back together again

Pete


« Last Edit: October 01, 2019, 06:35:21 AM by doubletop »
?To achieve anything in this game, you must be prepared to dabble on the boundary of disaster.? - Stirling Moss